ZIF receivers are known in the art. ZIFs directly demodulate RF (radio frequency) signals from an RF carrier to baseband, requiring a receive local oscillator (LO) input which when applied to internal mixers of the ZIF receiver down-converts the RF signals modulated on the common carrier frequency. A ZIF receiver includes baseband receiver pass band characteristics, including an undesirable null response within a narrow band of frequencies including zero Hz. The null response of the receiver pass band is commonly a consequence of AC coupled circuitry within the receiver intended to eliminate DC levels occurring in ZIF receiver internal in-phase and quadrature circuits associated with demodulation. Additional nulling circuitry may also be provided to balance any DC offset levels in the in-phase and quadrature circuits to prevent distortion of the demodulated output of the ZIF receiver.
This can be a problem, e.g., in a typical wireless system like a local area network (LAN) that has signaling patterns modulated onto a transmitted signal used to initiate a communications function (e.g., a dotting sequence). Modulating the carrier frequency with such patterns will generate spectral components having discrete frequencies at significant fractions (e.g., 1/2) of the system data rate. This is in contrast to the normal data modulation which is random and imposes a quasi-continuous, shaped spectrum on the carrier. With signaling pattern modulation, any strong carrier spectral content near the receive local oscillator frequency is down converted to a frequency near DC. Because typical system frequency errors are compatible with system bandwidths and data rates, this is significant for the center carrier frequency component. In a transient situation where a modulated carrier having such a spectrum is suddenly applied to the ZIF receiver, the spectrum initially passes through to the ZIF detector and is properly demodulated. However, after a period of time determined by both the high-pass corner of the aforementioned AC coupling circuitry and the time constant of the DC offset nulling circuitry, the carrier frequency component near DC is significantly attenuated. This can cause severe distortion in the demodulated signal pattern--generally rendering it useless for purposes such as sync and clock recovery.
Another common application affected is switched antenna diversity reception, implemented to provide the best available received signal by having two antennas alternately sampled over several repeated cycles. The sampling dwell time on each antenna is sufficient to permit an evaluation of the quality of the signal received on that antenna. The evaluation can be a simple signal strength measurement, or more complex evaluation such as an error measurement. The sampling dwell time typically includes several cycles of the aforementioned signaling pattern and can be a significant fraction of the transient response time associated with the ZIF null response. Under these timing conditions, and with the downconverted carrier frequency near DC, the antennas are switching between stronger and weaker signals, effectively presenting the ZIF with two DC levels resembling a DC-offset square wave. The equivalent high-pass action of the ZIF baseband response removes the DC offset which causes both the stronger and weaker signals to appear to have the same strength or quality so that a correct diversity decision is difficult or cannot be made.
Thus, a signal having a strong carrier frequency spectral component within the null response of the ZIF receiver can either have its modulated information destroyed or it can cause the receiver to be "blind" to significant differences in signal strength. One technique which has been employed to prevent carrier spectral content from causing such effects involves off-setting the receiver local oscillator frequency from the carrier frequency by an amount greater than the receiver half-bandwidth during transceiver receive mode (see Gehring, et. al., U.S. Pat. No. 4,944,025). This prevents down converted carrier spectral content from coinciding with the receiver null, but at the expense of receiver sensitivity and selectivity due to increased receiver noise bandwidth.
A further problem may arise in single frequency duplex transceivers, i.e., systems that transmit and receive RF signals using a common nominal carrier frequency. The problem arises during the transmit mode of the transceiver. For example, in non-voice data communications systems, it is typically necessary to minimize the transmit-to-receive turnaround time due to system throughput requirements. Unfortunately, with the transmitter operating on the same frequency as the receiver, enough DC spectral content may be generated by the down conversion to create a null-related transient desensitization condition--even though random data is being modulated and no strong carrier spectral component exists. Large amounts of isolation between the transmitter and receiver would be necessary to prevent transmit carrier spectral content from coupling to the receiver, but this may be difficult to achieve in practice. Methods of preventing desensitization which involve removing power from the receiver, disconnecting the baseband filters and holding their operating state, or shifting the frequency of the receive local oscillator to some other frequency during the transmit mode can unfortunately involve excessively long periods of time to re-establish stable operating conditions associated with the ZIF receiver and the receive local oscillator in receive mode.
To beneficially employ ZIF receiver advantages, particularly in high-throughput data communications systems, a need remains for an improved receiver architecture, including one which both prevents ZIF null-related responses from undermining receive performance, and permits fast transition from transmit mode to receive mode.